The larvae act as a midge thermometer, accurately recording the magnitude of past climate change, which could help scientists improve the forecasting ability of climate models.

If you’ve taken a dip in a lake anywhere in the world, chances are you’ve shared the water with the larvae of chironomids, or non-biting midges. The abundance of each midge species in a lake is directly related to summer air temperature at that location.

Each midge species thrives in a specific temperature range and, crucially, the winged adults respond quickly when climatic conditions are not ideal by shifting their distribution across the landscape. This means we can use assemblages of midge larvae, identified from their heads preserved in lake sediments, as a proxy for air temperature in the past - this is the basis of the midge thermometer.

Midges are one of the best temperature proxies that can be used to calibrate forecasts of temperature change.

Identified by dental records

As midge larvae grow they regularly shed their heads, which are then deposited and preserved in lake sediments. A 1cm slice of lake sediment could contain hundreds of individual midges’ remains.

Brooks explains:

‘It’s essential that we identify the midge species correctly. By studying the Museum’s huge chironomid collection we can confidently identify species by the characteristic shape and structure of their mouthparts.'

For the midge thermometer to work scientists also need to know the optimum summer air temperature at which each midge species thrives. To work this out Brooks collaborated with Professor John Birks at Bergen University to plot the current distribution and abundance of 150 midge species in 157 lakes across Norway against the current air temperature. The temperature at which each species is most abundant is taken as its optimum.

To test the accuracy of the midge thermometer Brooks and Birks compared the midge-inferred temperature with the measured temperature at each lake. The midge thermometer predicted the observed temperature for each lake with an error of just 1°C, proving that it is a highly accurate tool with great potential for studying palaeoclimates.

Taking Europe’s temperature

We know from ice core records and other temperature proxies that some of Earth’s most dramatic fluctuations in climate took place during the late glacial to early Holocene period, ~15,000-10,000 years ago. Understanding this kind of rapid and high-amplitude climatic change is especially valuable for scientists trying to predict how climate will change in the future.

The late glacial, which began around 15,000 years ago, marked the end of the last ice age, when the ice started to withdraw and temperatures across Europe increased rapidly. This warm period was followed by a steady decline punctuated by cold oscillations, before a rapid descent into the Younger Dryas, a 1,000-year-long period of arctic conditions. Then, around 11,700 years ago, there was another period of rapid warming, which brought us into the current epoch, the Holocene.

While scientists are confident in their knowledge of climate trends during this period, they are less certain about the absolute temperatures experienced in different regions and the factors that caused climate fluctuations. This is where the midge thermometer comes in.

Brooks says:

'We know that the European climate has been influenced in the past by changes in the extent of ice sheets, sea ice and the Atlantic Ocean circulation. As an independent indicator of absolute temperature, the midge thermometer allows us to resolve regional temperature trends across Europe and determine the relative influence of these climate drivers.'

Brooks and his colleagues used midge records from across western Europe to plot temperature change during the late glacial to early Holocene. The results showed strong north-south and east-west trends in temperature gradient. These reflect the relative influence at different times of the Scandinavian ice sheet in the north and the fluctuating current of the Atlantic Ocean in the west.

Microscope image of Dicrotendipes, a species of non-biting midge that indicates warm air temperatures

These studies allow us to check previous estimates of air temperature change, while telling us something new about regional trends across Europe and how climate may be affected in the future.

Putting climate models to the test

Along with determining palaeoclimates, the midge thermometer is a useful tool for assessing climate models' ability to accurately predict future fluctuations.

Brooks explains:

‘The only way to validate the climate models is to see how good they are at hindcasting - estimating changes in the past. You need data from independent sources to do that, and midges are one of the few temperature proxies that can provide it.'

In 2014 Brooks and a group of European collaborators used the midge thermometer to test the ECHAM-4 Atmospherical General Circulation Model, a tool used for global climate forecasting. They compared midge-inferred July temperatures across Europe from 15,000-11,000 years ago with hindcasts of temperature changes produced by the ECHAM-4.

The team found that the overall cooling and warming trends across Europe were well matched, suggesting that the driving mechanisms of climate used in the model are correct. However, the climate model's estimates of air temperatures during warm periods were about 2°C higher than those of the midge thermometer. This suggests that more work needs to be done to ensure that climate models do not overestimate predicted temperature increases.

Brooks says:

‘The European climate is influenced by multiple forcing mechanisms that vary both spatially and temporally. If we are to understand how this complex system of drivers will influence climate in the future, we need tools that can help us calibrate forecasts at a regional scale. The midge thermometer has great potential to help us do that.'